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Severe Combined Immunodeficiency (SCID) is a group of genetically inherited disorders which cause severe deficiencies in the immune system. It is caused by a genetic defect that causes an absence of T-lymphocytes, the white blood cells made in the bone marrow which play the main role on fighting infection.
This in turn causes a deficiency of B-lymphocytes, which are the cells that produce antibodies in response to foreign invaders. As a result, the immune system does not function properly, and is unable to fight of infection.
SCID affects around 1 in 100,000 live births; babies born with SCID effectively have no immune system, and are therefore highly vulnerable to infection, as their bodies do not have the cellular mechanisms needed to fight off foreign antigens.
If untreated, these children will usually die within a year from infections such as pneumonia, meningitis and chicken pox.
There are several forms of SCID, the most common of which is caused by a mutation on a gene located on the X chromosome and affects only males; this is referred to as x-linked severe combined immunodeficiency (X-SCID).
In the past, children born with SCID have had to be confined due to their vulnerability to infection, hence the term ‘bubble baby syndrome’. In more recent times, treatment options have improved. The current best treatment option is a bone marrow transplant, which has a high level of success as long as there is a well-matched donor such as a sibling. Unfortunately, good matches can only be found in one third of cases, and success with less well matched donors is considerably lower.
Gene therapy involves inserting copies of a missing gene or genes into a patient’s cells. In SCID, samples of T-cells with defective genes are isolated, and working copies of the gene are inserted using a modified virus as a vector. These modified cells are then returned to the patient’s bone marrow, where they grow rapidly and return some, or all, immune system functionality.
The use of gene therapy to treat X-SCID carries with it a risk of leukaemia – a trial in Paris led to 4 children (out of 11 treated) developing the disease, one of whom died while the others were treated and recovered. The leukaemia was caused by the introduced gene being implanted next to, and switching on, an oncogene (cancer-causing gene). The three children who were treated for leukaemia made a full recovery and now have working immune systems.
Despite being a serious illness, leukaemia can be successfully treated in more than 80 per cent of cases, suggesting that gene therapy is safer and carries a higher rate of success than bone marrow transplantation.
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